The present invention relates to oil and gas well drilling equipment and methods of obtaining core samples.
Wells are generally drilled to recover natural deposits of hydrocarbons and other desirable, naturally occurring materials trapped in geological formations in the earth""s crust. A slender well is drilled into the ground and directed to the targeted geological location from a drilling rig at the surface. In conventional xe2x80x9crotary drillingxe2x80x9d operations, the drilling rig rotates a drillstring comprised of tubular joints of steel drill pipe connected together to turn a bottom hole assembly (BHA) and a drill bit that is connected to the lower end of the drillstring. During drilling operations, a drilling fluid, commonly referred to as drilling mud, is pumped and circulated down the interior of the drillpipe, through the BHA and the drill bit, and back to the surface in the annulus.
Once a formation of interest is reached in a drilled well, drillers often investigate the formation and the deposits therein by obtaining and analyzing representatives samples of rock at multiple locations in the well. Each representative sample is generally cored from the formation using a hollow coring bit, and the sample obtained using this method is generally referred to as a core sample. Once the core sample has been transported to the surface, it may be analyzed to assess the reservoir storage capacity (porosity) and the flow potential (permeability) of the rock material that makes up the formation, the chemical and mineral composition of the mineral deposits residing in the pores of the formation, and to measure the irreducible water content of the rock material. The information obtained from analysis of the sample is used to design and implement well completion; that is, to selectively produce certain economically attractive formations from among those accessible by the well. Once the driller has decided upon a well completion plan, all formations except those specifically targeted for production are isolated from the target formations, and the deposits within targeted formations are selectively produced through the well.
Several coring tools and methods of obtaining core samples have been used. Conventional coring occurs where the drillstring is removed from the wellbore and a rotary coring bit having a hollow interior for receiving the cut core sample is run into the well on the end of the drillstring. The core obtained using conventional coring is taken in the path of the drillwell; that is, the conventional coring bit is substituted in the place of the drill bit and the portion of the formation in the path of the well is sampled instead of ground up and removed from the well by the mud flow. Sidewall coring occurs where the core sample is taken from the bore wall of the drilled well.
There are generally two types or categories of sidewall coring tools, rotary and percussion. Rotary coring is generally performed by forcing an open, exposed end of a hollow cylindrical coring bit against the wall of the bore hole and rotating the coring bit against the formation. The coring tool is generally secured against the wall of the bore hole or well with the rotary coring bit oriented towards the opposing wall of the bore adjacent to the formation of interest. The coring bit is generally deployed from the coring tool and against the bore wall by an extendable shaft or other mechanical linkage that is also used to rotate the coring bit against the formation. The coring bit generally has a cutting edge at one end, and the coring tool generally imparts rotational and axial force to the coring bit through the shaft or other mechanical linkage to cut the core sample. Depending on the hardness and degree of consolidation of the target formation, the core sample may also be obtained by vibrating or oscillating the open and exposed end of a hollow bit against the wall of the bore hole or even by application of axial force alone. The cutting edge of the bit is usually embedded with carbide, diamonds or other hard materials with superior hardness for cutting into the rock portion of the target formation.
As the core sample is cut and the bit advances into the formation, the core sample is received within the hollow barrel of the coring bit. After the desired length of the core sample or the maximum extension of the coring bit is achieved, the core sample is generally broken from its remaining interface or connection with the formation by displacing the coring tool and, through displacement of the linkage used to extend and impart motion to the coring bit, tilting the coring bit and the protruding core sample within the bit from their cored orientation. The core sample is usually broken free at the remaining interface with the formation by displacement of the coring tool within the wellbore, thereby imparting a breaking moment to the core sample through the coring bit. After the core sample is broken free from the formation, the hollow coring bit and the core sample received within the barrel of the coring bit are retrieved into the coring tool through retraction of the coring shaft or mechanical linkage that is used to deploy the coring bit to, and to rotate the coring bit against, the formation. Once the coring bit and the core sample have been retracted to within the coring tool, the retrieved core sample is generally ejected from the coring bit to allow use of the coring bit for obtaining subsequent samples at the same or other formations of interest. When the coring tool is retrieved to the surface, the recovered core sample is transported within the coring tool for analysis and tests. The present invention is designed for use with this type of coring process.
The second common type of coring is percussion coring. Percussion coring uses cup-shaped percussion coring bits that are propelled against the wall of the bore hole with sufficient force to cause the bit to forcefully enter the rock wall such that a core sample is obtained within the open end of the percussion coring bit. These bits are generally pulled from the bore wall using flexible connections between the bit and the coring tool such as cables, wires or cords. The coring tool and the attached bits are returned to the surface, and the core samples are recovered from the percussion coring bits for analysis.
The retrieval and analysis of core samples in their undamaged condition provides valuable geologic information that improves analysis and reservoir management. There are some problems with conventional coring equipment that result in loss or damage to core samples, and a related loss of valuable information.
Throughout the process of cutting and retrieval of the core sample using conventional coring equipment, the open end of the coring bit remains open. Unfortunately, the core sample is often lost through the open end of the coring bit while the coring bit and the core sample are being retrieved to within the coring tool. This risk of loss of the cut core sample from the open end of the coring tool is increased when the cutting zone from which material is removed during the cutting process is larger, as may result using non-conventional coring bits, such as with brush bits comprising a plurality of rigid bristles used to cut the formation.
Also, the coring process itself can cause damage to the core sample during coring and after it is broken free of the formation face. In the process of applying a breaking moment to the core sample to break it free of the formation, the core sample is often broken too far from the interface with the formation, resulting in a shorter and less useful core sample. Also, the core sample may be broken and eroded by xe2x80x9ctumblingxe2x80x9d within the hollow barrel of the rotating coring bit. Unconsolidated core samples may be damaged upon mechanical ejection from the coring bit to storage bins within the coring tool, or even upon removal from the storage bins at the surface.
What is needed is a device and method of breaking the core sample free from the formation without the necessity of displacement of the entire coring tool and without imparting excessive force to the linkage that extends and rotates the coring bit. What is needed is a device that secures the cut core sample within the coring bit to prevent loss of the cut core sample from the open end of the coring bit during the retrieval stage of the coring process. What is needed is a device that enables drillers to obtain a greater quantity of cut core samples in close to their original, undamaged conditions. It is preferred that the device and method of improving recovery of cut core samples be useful with existing coring tools.
The present invention provides a core retaining sleeve for improved recovery and retention of core samples from subsurface geologic formations, and a method of recovering cut core samples cut from a subsurface geologic formation. The core retaining sleeve uses one or more retaining xe2x80x9cfingersxe2x80x9d which, when deployed, impose one or more obstacles preventing loss of the cut core sample from the open end of the hollow interior of the coring bit. The core retaining sleeve is designed to reside within or around the coring bit without interfering with the cutting process of the coring bit during cutting of the core sample, and to be deployed radially outwardly from the well center to its retaining position. As the core retaining sleeve is deployed to capture the core sample, the retaining finger(s) are actuated to sever the core sample from the formation or to obstruct the loss of the core sample from the open end of the coring bit if the core sample is already severed. The core sample is thereby trapped within the hollow interior barrel of the coring bit by the actuated retaining finger(s) of the core retaining sleeve thereby preventing loss of the core sample from the open end of the coring bit during retrieval of the coring bit and the core sample to within the coring tool. The core retaining sleeve may remain stationary relative to the coring bit or it may rotate with the coring bit. Optionally, the core retaining sleeve may have internal or external grooves or channels to assist in removal of cuttings and debris or to impart a secondary reaming or boring effect to the brush bit.
The present invention also provides a tilting wedge that, when deployed against the proximal (coring tool) end of a cut core sample, imparts a breaking moment to the cut core sample sufficient to break it free from the remaining interface with the formation. Optionally, the tilting wedge may provide for improved retention of the core sample within the coring tool to prevent loss during retraction of the core sample to within the coring tool.
The present invention also relates to an apparatus for obtaining a core sample comprising a coring bit, a core retaining sleeve and an actuator. The coring bit has an interior wall and one or more stationary guide members formed on the distal end of the interior wall. The core retaining sleeve is in concentric alignment within the coring bit, the sleeve having one or more closeable retaining fingers at a distal end and defining a chamber for storing the core sample. The actuator forces the one or more closeable retaining fingers against the one or more stationary guide members to radially deflect the retaining fingers to a closed position. The apparatus may have a plurality of core retaining sleeves, or at least one core retaining sleeve. The apparatus may further include means for selectively positioning each of the core retaining sleeves within the coring bit to obtain a different core sample.
The present invention also relates to a method for obtaining a core sample. The method comprises cutting a core sample in a sidewall of a wellbore, disposing a core retaining sleeve around the core sample; detaching the core sample from the sidewall, and capturing the core sample within the core retaining sleeve. The method may further comprise repeating the steps at other locations in the wellbore using additional core retaining sleeves.